Evaluating Efficiency of UVC Air Sanitizing Devices and Systems
To choose a UVC mobile or HVAC air sanitizing device or system with a high degree of confidence, it is important to be able to make a quantitative evaluation based on well-founded metrics.
An effective air sanitizing system must kill viruses in the air effectively and rapidly after they are released into indoor air space via aerosols produced by coughing, sneezing or even talking. Our metrics focus on the dynamics of virus release and spread in an indoor air space, while putting a premium on the device's ability to provide rapid and effective disinfection. In developing the N5Air metrics, we consulted a recent nature.com publication.
The parameters used for calculating the efficiency of a UVC-based air sanitizing system include:
1. Residence time in the UVC sanitizer chamber,
2. UVC power,
3. Air flow rate through the UVC sanitizer chamber.
In the calculations, the experimental value of exposure lethal to virus (nature.com) is also used (thereafter, EXPOSURE). We will refer to the system parameters as RESIDENCE TIME, POWER, and AIR FLOW.
RESIDENCE TIME tells how long an aerosol particle carrying viruses will stay within the chamber being exposed to the lethal for them UVC light. If the RESIDENCE TIME, given the POWER is not sufficient to reach the value of EXPOSURE, some of the viruses will survive until they get back in the chamber later. In an effective system, EXPOSURE is always reached.
POWER indicated what UVC light power is available to kill viruses. As mentioned above, UVC POWER must be high enough for the viruses get the EXPOSURE during the RESIDENCE TIME. Again, if POWER is not high enough, some viruses will survive the passage through the chamber.
Finally, the value of the AIR FLOW provided by the UVC air sanitizing system determines its ability to rapidly create and maintain a safe virus-free zone given the total size of the INDOOR SPACE (m3).
The N5Air method for metrics calculation predicts the system effectiveness, EFFECTIVENESS as a product of two factors, POWER and the AIR RECIRCULATION FACTOR:
EFFECTIVENESS = (POWER / CHAMBER VOLUME) x AIR RECIRCULATION FACTOR
In the above expression:
CHAMBER VOLUME is the volume of the device’s sanitizer chamber (m3) containing the UVC light and
AIR RECIRCULATION FACTOR is the ratio of the air flow rate through the sanitizing device, AIR FLOW (m3/sec) and the total volume of the indoor air space being sanitized, INDOOR SPACE:
AIR RECIRCULATION FACTOR = AIR FLOW / INDOOR SPACE.
Note that (1 / AIR RECIRCULATION FACTOR) is the time in seconds required for circulating the air in the entire INDOOR SPACE through the sanitizing device one time. A larger AIR RECIRCULATION FACTOR indicates that the air sanitizing system has a faster response in dynamic situations.
In our calculations, the quantitative information on lethality of UVC light to Sars-Cov2 viruses presented in nature.com was used in calculating the value of EXPOSURE. This EXPOSURE is compared to the UVC exposure actually delivered by each particular system.
N5Air uses the above method in predicting the effectiveness metrics for evaluating and comparing various UVC mobile and HVAC air sanitizing systems.
Virus-free indoor air space